1. Field of the Invention
The present invention relates to an optical pickup device for projecting light beam to an optical recording medium so that record of information or reproduction of information can be performed. In particular, the present invention relates to a structure of a diffraction element that is disposed in an optical system of the optical pickup device for generating diffracted light.
2. Description of Related Art
At present, optical recording media including a compact disc (hereinafter referred to as a CD) and a digital versatile disc (hereinafter referred to as a DVD) are widely available. In addition, recent years, in order to increase recording capacity of the optical recording medium, studies about high density recording on the optical recording medium have been proceeding. For example, high density optical recording media including an HD-DVD and a Blu-Ray Disc (hereinafter referred to as a BD) that are high quality DVDs are being available. When such an optical recording medium is read or written, an optical pickup device is used, which projects a light beam to the optical recording medium for writing and reading information.
An optical system of the optical pickup device is provided with a diffraction element that diffracts the light beam for various purposes. For example, the optical pickup device is required to adjust a focal point of the light beam emitted from a light source on a recording surface of the optical recording medium constantly and to make a position of a light beam spot follow a track formed on the optical recording medium. Therefore, some optical pickup devices are equipped with a diffraction element disposed in its optical system for obtaining a servo signal such as a focus error signal or a tracking error signal.
For example, Japanese patent No. 3560805 or JP-A-2004-327005 discloses a structure in which a diffraction element is disposed in an optical pickup device for the purpose described above. The structure described in Japanese patent No. 3560805 includes a liquid crystal panel that works as a diffraction element and is disposed at the vicinity of the semiconductor laser. This liquid crystal panel separates the light beam emitted from the semiconductor laser into a main beam (0 order light) and a sub beam (±1 st order light) so that a tracking control can be performed by using a so-called three-beam method. In addition, the liquid crystal panel is used as the diffraction element, and a power supply for applying a voltage to the liquid crystal panel is turned on and off so that a diffraction function can be switched.
Furthermore, in the optical pickup device that is described in JP-A-2004-327005 is provided with a common optical path for leading light beams emitted from a plurality of light sources having different wavelengths to an optical recording medium, and a diffraction element is disposed on the common optical path so that ±1 order diffracted light is obtained from the light beam entering the common optical path. In this structure, a tracking error signal is obtained by the arrangement of the diffraction element and is used for the tracking control. Moreover, in the optical pickup device that is described in JP-A-2004-327005, the diffraction element has two types of diffraction grating areas formed on the same surface, for example, because it is necessary to support two light beams having different wavelengths.
One of other purposes of arranging a diffraction element in an optical pickup device is to downsize the optical pickup device that can support a plurality of types of optical recording media. For example, JP-A-2004-219750 discloses such an optical pickup device. In this device, a light source and a light receiving portion are housed in a single case, and the diffraction element is used for switching an optical path of light projected to an optical recording medium and reflection light from the optical recording medium. A problem that may occur when the light source and the light receiving portion are housed in the single case is solved by a role of this diffraction element, so a size of the optical pickup device can be reduced. At this point, the diffraction element disclosed in JP-A-2004-219750 is made up of liquid crystal and that two diffraction elements are arranged so as to support two light beams having different wavelengths.
However, the optical pickup devices disclosed in above described Japanese patents No. 3560805, JP-A-2004-327005, and JP-A-2004-219750 are not sufficient from viewpoints described below. First, the diffraction element that is provided to the optical pickup device described in Japanese patent No. 3560805 has a disadvantage in the case where the optical pickup device is required to a plurality of optical recording medium. In this case, the diffraction element is necessary for each of light sources for each of optical recording media, so a size of the optical pickup device increases resulting in an increase of cost. Recently, the optical pickup device is required to support a plurality of optical recording medium. Therefore, the above-mentioned disadvantage may become an important problem.
Furthermore, the diffraction element that is provided to the optical pickup device described in JP-A-2004-327005 has a structure in which two different diffraction grating areas are formed, and a light beam having a wavelength that is diffracted by one of the diffraction grating areas is not diffracted by the other diffraction grating area. Manufacturing the diffraction element having the above-mentioned structure is technically difficult and requires a lot of cost if it is possible. Therefore, as a matter of fact, when the light passes through the diffraction grating area that should not diffract the light, undesired diffracted light that is called stray light may be generated in many cases. If the stray light is generated, many problems may occur, which includes a decrease of utilization factor of light, adverse effect to a reproduction signal and a servo signal, and deterioration of a signal.
Concerning these problems, it is difficult to separate a light beam for a CD from a light beam for a HD-DVD and a BD without generating the stray light because the former has a wavelength (e.g., 780 nm) and the latter has another wavelength (e.g., 405 nm), which are different from each other by a ratio substantially twice (or a ratio of integer multiple). This could be a serious problem in development of the optical pickup device supporting a HD-DVD and a BD that are developed actively recent years.
In addition, the diffraction element described in JP-A-2004-219750 is not sufficient for downsizing the optical pickup device, because it is necessary to arrange a plurality of diffraction elements for supporting a plurality of light beams having different wavelengths. Furthermore, there is also a problem of an increase of components resulting in an increase of manufacturing cost.